Thermal management in semiconductor devices and circuits is a critical design element in any manufacturable and cost-effective electronic and optoelectronic product, such as light generation and electrical signal amplification. The goal of efficient thermal design is to lower the operating temperature of such electronic or optoelectronic devices while maximizing performance (power and speed) and reliability. Examples of such devices are microwave transistors, light-emitting diodes and lasers. Gallium nitride (GaN), aluminum nitride (AlN) and other wide-gap semiconductors are commonly used in both power electronics and visible-light generating optoelectronics. Gallium nitride material systems give rise to microwave transistors with high-electron mobility and high breakdown voltage suitable for use in high power applications. GaN is also used in manufacturing of blue and ultraviolet lasers and light-emitting diodes. In spite of the high-temperature performance, GaN and related ternary compounds exhibit relatively low thermal conductivity of the substrates commonly used for growth of GaN. This deficiency is most pronounced in high-power microwave and millimeter-wave transistors and amplifiers where reduced cooling requirements and longer device life, both benefiting from lower junction temperature, are in critical demand. One of the most efficient and advanced technologies for thermal management high-power devices relies on placing the active layers of GaAs, InP, Si, or GaN on top of a highly thermally conductive heatsink and ensuring that the distance between the active epilayers and the heatsink is minimized. Shortening the distance heat has to flow from the heat source to the highly thermally conductive heat spreaders and heatsink lowers the overall thermal resistance. Diamond is the most thermally conductive substance known to man and the semiconductor industry has been employing diamond heat-sinks and heat spreaders for improved thermal management since the commercialization of synthetic diamond by chemical-vapor deposition in the 1980-ies. There is more than one way of placing active epilayers on top of diamond heatsinks and heat-spreaders. In particular, epilayer transfer or atomic attachment technologies in which several micrometer epilayers are transferred onto diamond wafers are most promising methods for thermal management of power devices. In addition, there are other reasons for transferring epilayers to diamond. For example, diamond hardness and high breakdown field are other reasons one combines a semiconductor and diamond in one device. The final result of such an integration is a semiconductor-on-diamond wafer and ultimately a chip.
Diamond wafers are manufactured by chemical vapor deposition (CVD) by one of three methods: plasma enhanced diamond CVD where the energy to dissociate the reactants comes from a microwave sources, hot-filament enhanced diamond CVD where the energy for dissociating gasses comes from a hot tungsten filament, and high voltage torch where ions are accelerated using a high DC voltage. Using these technologies, diamond wafers of standard diameters and thicknesses can be created. There is however one essential difference between working with diamond substrates and other common semiconductor substrates, such as, Gas, InP, and Silicon: diamond is hard to thin and polish (diamond grit is commonly used for thinning of other semiconductors), but most importantly, diamond wafer price increases with its thickness and hence it is difficult to justify using a diamond substrate thicker than is necessary both for thermal management purposes or chip handling purposes. The most significant consequence of this fact is that diamond wafers are not thinned during backside processing as for standard semiconductor wafers. Therefore, the thickness of the wafer at the start of the process is necessarily equal, or approximately equal, to the desired final chip thickness. For example, device processing on silicon, gallium arsenide, gallium nitride wafers occurs while these substrates are over 500 μm thick. Such thickness provides sufficient mechanical rigidity and low bow that commercial foundries can handle such wafers. At the end of the device processing, the wafers are thinned down to the desired chip thickness. The chip thickness, on the other hand, is determined by several constraints: optimal heat-spreading from the active device to the heatsink below the chip and ease of dicing are just two of the most important constraints. Since thinning down diamond wafers makes little economic sense, semiconductor-on-diamond wafers tend to be of the same thickness as the final chips, which is generally thinner than typical commercially available silicon or compound-semiconductor wafers. Since chip thicknesses and hence semiconductor-on-diamond wafers may range from tens of micrometers to several hundred micrometers, they are too thin for commercial foundry processing. First, handling such thin wafers with diameters larger 25 mm is exceedingly difficult and not practical in neither manual nor robotic/automated processing. Second, CVD-grown diamond wafers include internal stresses and exhibit bow that is several times larger than the common limit for wafer bow in commercial lithography tools. Industrial processing plants will generally not accept wafers with such attributes. Additionally, the diamond surface of the semiconductor-on-diamond wafer is often rough with average absolute roughness of several tens of micrometers which makes it more difficult to hold wafers down to a chuck using vacuum—a feature important in lithography processing. Similar problems appear when thru-vias are used on wafers.
There has been substantial effort in the industry to bond GaN epilayers to CVD diamond and to attach other compound materials to diamond to create highly thermally efficient electronic structures. As a result there is a need in the industry to enable the commercial processing of thin semiconductor-on-diamond wafers. This application discloses structures and methods to accomplish this goal.